Polarization independent devices such as optical circulators and isolators generally require separating the input beam having an unknown polarization state, into two orthogonally polarized sub-beams. These sub-beams are routed through the isolating elements of the device such as reciprocal and non-reciprocal rotators and are combined at an output end. However, if the beams are launched in a backwards direction non-reciprocal elements ensure that the light does not couple back into the input port. Rutile crystals, and other birefringent crystals are well known for the purpose of separating an input beam into two orthogonally polarized sub-beams thereby serving as a polarization beam splitter, or operated in an opposite direction as a polarization beam combiner. Within this specification the term polarization beam splitter is used however it should be understood, that the same device serves as a polarization beam combiner operated in reverse.
It has been typical, for light propagating within these crystals to be collimated, most often by a graded index (GRIN) lens. In this instance a relatively large crystal is required to ensure separation of two beams that have diameters typically as large as 350 μm. However, recently, it was discovered that very small crystals, about 1/50th the size of conventional crystals could be used with a non-collimated beam; using such small crystals substantially lessens the cost of manufacturing optical splitters/combiners, circulators or optical isolators.
A polarization beam combiner joins light from different inputs into one common port. This is commonly required, for example, for combining pump power from one or more lasers with an optical signal into an optical amplifier. A polarization splitter splits a beam of light into two sub-beams of orthogonal polarization components directed to two separate outputs. It is desired to provide isolation to prevent light from coupling back into the input port of the splitter. This is generally done by providing an external isolator, a pigtailing of polarization maintaining fiber, and a separate splitter.
Isolation for the combiner is more critical, as light coupled back to the laser source will cause damage to the laser. An external isolator is provided between the lasers and the combiner. However, laser pump power is expensive to provide, and using a separate isolator and combiner increases insertion losses.
It is an object of the present invention to provide a combiner, which can provide the isolation function and increase pump coupling efficiency. It is a further object to provide a smaller, more cost effective isolated polarization splitter/combiner using fewer parts and less polarization maintaining fiber.
A further disadvantage of prior art object space polarization beam splitters/combiners is that there is a difference in optical path length for the two separated orthogonal polarizations traveling through a birefringent crystal. Using birefringent crystals where the light propagating therethrough is not collimated, leads to an increase in insertion loss due to a defocusing or a need to compensate for the path length difference. The sub-beams follow a slow axis (extraordinary), and a fast axis (ordinary), which correspond to this difference in optical path length. It is typical after separating the beam into its two orthogonal polarization states through a rutile crystal to couple the light into two fiber ends. However, the two focus spots from the crystal do not lie on a same focal plane. This is due to the optical path length difference for the e-ray and the o-ray through the crystal. Generally pairs of optical fibers are held securely in a fixed manner in an optical fiber tube. If a tube is used to couple light from the crystal aligned at the focus spot of one of the beams, the other focus spot will not be in focus at the tube end, and light from either the e-ray or o-ray path will couple poorly.
It is a further object of this invention to provide an isolated beam splitter/combiner which lessens or obviates this optical path length difference, having substantially same optical path lengths for two split or combined beams propagating therethrough.